The present invention relates to a laser oscillator, especially to the laser oscillator suitable for ultrafine manufacturing such as an apparatus for manufacturing of semiconductor.
A miniaturized or enlarged figure of a drawing which is drawn on a mask is depicted on an image forming plane by transmitting the mask with light and focusing the image of the drawing of the mask on the image forming plane with a lens etc.
A laser is used as a light source of the process described above, and the process is utilized for ultrafine fabrication etc. As of the laser oscillator for the utilization, there are some which are disclosed in Japanese Patent Laid-Open No. 61-163681 (1986) (prior art 1) and Japanese Utility Model Laid-Open No. 62-49263 (1987) (prior art 2).
For avoiding a blurred image spatially and chronically in order to obtain stable focusing image on the mask in ultrafine fabrication, it is necessary to have a constant directivity chronically and no interference even though the directivity is poor.
Both of the prior arts described above are aimed to form laser having good directivity. The prior art 1 is for eliminating of light having poor directivity from output light with light collecting elements and an aperture mechanism for making the output light have good directivity. The prior art has a problem of low efficiency.
On the other hand, in the prior art 2, two light collecting elements and an aperture mechanism are installed at a confocal position between the laser medium which emits the laser continuously and the output mirror. As generating the laser light continuously, it is possible to have a constant directivity chronically, but because of forming a single mode (strength distribution like single Gauss distribution) wherein the number of strongness and weakness in strength of the beam (which is called as the order number of lateral mode) which is generated at a vertical cross section of light being formed in the oscillator is single, interference is strong and a stripe pattern is formed, and a problem that strength of the image is not uniform spatially is caused.